The green flash is an atmospheric optical phenomenon that occurs during the last seconds of sunset or the first seconds of sunrise, when the upper edge of the solar disc emits a brief flash of intense green colour lasting between 1 and 3 seconds. It was popularised by Jules Verne's novel The Green Ray (1882) and is often considered an elusive phenomenon, though under favourable conditions it is quite observable. The mechanism is based on atmospheric dispersion: the atmosphere acts as a weak prism, slightly separating the colours of sunlight. When the sun is at the horizon, its light passes through the maximum thickness of atmosphere. Blue and violet rays are scattered by the atmosphere (which is why they are not seen), red and orange form the lower part of the solar image, and green remains at the top. In the final moment before disappearing, only the green upper rim of the solar disc remains visible. Observing the green flash requires a very clean, distant horizon—the sea is ideal—stable atmosphere without excessive turbulence, and moderate refraction (neither too cold nor too warm a surface). Several types are classified: the inferior flash (most common, 1–2 seconds), the mock-mirage flash (longer lasting, associated with temperature inversions), and the extremely rare green ray proper (a beam rising above the sun). Atlantic coastlines offer excellent observation opportunities.